The present invention relates to a screw for plasticating plastic material and a method for mixing plastic material that has been at least partially plasticized and contains inhomogeneities.
Extrusion and injection molding processes form shapes from thermoplastic resin by softening or plasticating solid material to a molten state, shaping the product in an extrusion die or in a closed mold, and solidifying the shaped product by cooling. The properties of the final product depend to a major extent on the processing history of the material, and particularly on the thermal and physical homogeneity of the molten material prior to forming it, as well as the length of time the material is heated. For example, acid aldehyde formation in polyethylene teraphthalate (PET) type resins is directly related to the temperature, shear history and total cycle time. Accordingly, processing of a homogeneous melt at the lowest possible temperature, with minimized shear and time is a major concern in the injection molding and extrusion industries.
Over the years, a great deal of effort has been spent on designing the screws used to plasticate the material to be molded. As a result, the prior art is replete with a wide variety of differently configured plasticating screws. U.S. Pat. Nos. 2,753,595 to Dulmage and 3,006,029 to Saxton describe plasticating screws that employ distributive mixing elements to break down agglomerations or inhomogeneities in the melt. U.S. Pat. Nos. 3,687,423 to Koch et al.; 3,701,512 to Schippers et al.; 3,858,856 to Hsu; 3,870,284 to Kruder; 3,941,535 to Street; 4,000,884 to Chung; 4,079,463 to Miller; 4,128,341 to Hsu; 4,215,978 to Takayama et al.; 4,639,143 to Frankland, Jr.; and 4,925,313 to Nunn illustrate some other types of plasticating screws that employ dispersive mixing. Still another type of plasticating screw is shown in abandoned U.S. patent application Ser. No. 757,042, filed Jan. 5, 1977. There is disclosed in this abandoned application a screw provided with a helical flight and a valley wherein the bottom surface of the valley is divided into a plurality of parallel strip shaped portions having wavy contours with a predetermined phase difference therebetween which form a plurality of cross points between the wavy contours along the dividing plane.
New developments in plastic resin compositions now allow for PET type products to be used under relatively high temperatures, in so called "hot-fill" applications. In order to maintain physical properties at these higher temperatures, a polymer blend of PET and polyethylene napthalate (PEN) is normally used. However, PEN and PET are immiscible, so distributive mixing is required. Conventional PET screws cannot successfully process these new copolymer blends.